Christopher Cheng

Adjunct Professor, Surgery - Vascular Surgery

Bio

Bio

Currently, I split my time between the medical device industry and academic research. For my day job, I am the CEO of a medical device start up developing a novel implantable device to treat gallstone disease. At Stanford, my research is focused on the interaction between vascular implants and the dynamic anatomy. I advise companies on device design, biomechanics, mechanical testing, preclinical and clinical trials, regulatory affairs, and clinical affairs.

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Research & Scholarship

Current Research and Scholarly Interests

Our research laboratory focuses on understanding the mechanics of the cardiovascular system, especially with respect to interactions between medical devices and the dynamic cardiovascular environment. We use medical imaging, 3D geometric modeling, and custom deformation quantification techniques to investigate disease processes and medical device performance. We are interested in the dynamics of the heart, aorta, and peripheral vasculature, and are always seeking ways to apply our research to current and emerging therapies. While our research pursuits seek to add to the fundamental understanding of cardiovascular biomechanics, all of our projects are directly related to improving medical device design, evaluation, regulation, and their use in clinical practice.

Abstract

Aneurysm regression and target vessel patency during early and mid-term follow-up may be related to the effect of stent-graft configuration on the anatomy. We quantified geometry and remodeling of the renal arteries and aneurysm following fenestrated (F-) or snorkel/chimney (Sn-) endovascular aneurysm repair (EVAR).Twenty-nine patients (mean age, 76.8 ± 7.8 years) treated with F- or Sn-EVAR underwent computed tomography angiography at preop, postop, and follow-up. Three-dimensional geometric models of the aorta and renal arteries were constructed. Renal branch angle was defined relative to the plane orthogonal to the aorta. End-stent angle was defined as the angulation between the stent and native distal artery. Aortic volumes were computed for the whole aorta, lumen, and their difference (excluded lumen). Renal patency, reintervention, early mortality, postoperative renal impairment, and endoleak were reviewed.From preop to postop, F-renal branches angled upward, Sn-renal branches angled downward (P

Abstract

Visceral arteries are commonly involved in endovascular repair of complex abdominal aortic aneurysms (AAAs). To improve repair techniques and reduce long-term complications involving visceral arteries, it is crucial to understand in vivo arterial geometry and the deformations due to visceral organ movement with respiration. This study quantifies deformation of the celiac, superior mesenteric (SMA), and renal arteries during respiration and correlates the deformations with diaphragmatic excursion.Sixteen patients with small AAAs underwent magnetic resonance angiography during inspiratory and expiratory breathholds. From geometric models of the aorta and visceral arteries, vessel length, branch angle, curvature, and positions were computed, along with degree of diaphragmatic excursion as indicated by kidney translation.From inspiration to expiration, the celiac artery exhibited axial shortening of 4.8 ± 6.4% (P 0.50). Correlation of the LRA with the left kidney was greater than that of the RRA with the right kidney.The celiac artery exhibited less branch angle change, and greater axial and curvature deformations than the other visceral arteries, due to the vicinity to the liver and influence of the median arcuate ligament. Correlation between visceral arteries and kidney translations revealed that diaphragmatic excursion affects vessel mobility. Weaker correlation of the RRA to the right kidney indicates mechanical shielding from the inferior vena cava.

Abstract

The durability of stent grafts may be related to how procedures and devices alter native anatomy. We aimed to quantify and compare renal artery geometry before and after fenestrated (F-) or snorkel/chimney (Sn-) endovascular aneurysm repair (EVAR).Forty patients (75 ± 6 years) underwent computed tomographic angiography before and after F-EVAR (n = 21) or Sn-EVAR (n = 19), with a total of 72 renal artery stents. Renal artery geometry was quantified using three-dimensional model-based centerline extraction. The stented length was computed from the vessel origin to the stent end. The branch angle was computed relative to the orthogonal configuration with respect to the aorta. The end-stent angle was computed relative to the distal native renal artery. Peak curvature was defined as the inverse of the radius of the circumscribed circle at the highest curvature within the proximal portion from the origin to the stent end and the distal portion from the stent end to the first renal artery bifurcation.Sn-renals had greater stented length compared to F-renals (P < .05). From the pre- to the postoperative period, the origins of the Sn-left renal artery and right renal artery (RRA) angled increasingly downward by 21 ± 19° and 13 ± 17°, respectively (P < .005). The F-left renal artery and RRA angled upward by 25 ± 15° and 14 ± 15°, respectively (P

Abstract

This study quantified the geometry and respiration-induced deformation of abdominal branch vessels and stents after fenestrated (F-) and snorkel (Sn-) endovascular aneurysm repair (EVAR).Twenty patients (80% male; mean age, 75.2 ± 7.4 years; mean aneurysm diameter, 6.2 ± 1.8 cm) underwent computed tomography angiography during inspiratory and expiratory breath hold protocols after F-EVAR (n = 11) or Sn-EVAR (n = 9). Centerlines for the aorta and visceral vessels were extracted from three-dimensional models. Branch angles were computed relative to the orthogonal plane at the branch ostia, and end-stent angles of the left renal artery (LRA) and right renal artery (RRA) were computed relative to the distal stent orientation. The radius of peak curvature was defined by the circumscribed circle at the highest curvature.Sn-renal branches were more downward-angled than F-renal branches (P < .04). At the distal ends of the RRA stents, Sn-RRAs were angled greater than F-RRAs (P < .03) and had a smaller radius of peak curvature (P < .03). With expiration, the end-stent angle of Sn-LRAs increased by 4° ± 4° (P < .02) and exhibited a significant reduction of radius of curvature (P < .04). The unstented celiac arteries were more downward-angled (P < .02, inspiration), with a smaller radius of curvature (P < .00001), than the unstented superior mesenteric arteries. With expiration, the celiac arteries angled upwards by 9° ± 9° (P < .0005), which was greater than the superior mesenteric arteries (P < .03). At a median postoperative follow-up of 12.6 months (range, 1.0-37.1 months), branch vessel patency was 100%, serum creatinine levels remained stable, and one reintervention was required for a type III endoleak at the main body-LRA stent interface.Sn-renals were angled more inferiorly at the branch and more angulated at the stent end than F-renals due to stent placement strategies. Sn-LRAs exhibited a significant change in end-stent angle and curvature during respiration, a finding that may compromise long-term durability for parallel stent graft configurations. Further investigation is warranted to better optimize anatomic, patient, and branch vessel stent selection between fenestrated and snorkel strategies and their relationship to long-term patency.

Abstract

To quantify aortic arch geometry and in vivo cardiac-induced and respiratory-induced arch translations and arch branch angulations using three-dimensional geometric modeling techniques.Scanning with electrocardiogram-gated computed tomography angiography during inspiratory and expiratory breath holds was performed in 15 patients (age, 64 y ± 14) with thoracic aortic aneurysms or dissections. From the lumen models, centerlines of the thoracic aorta, brachiocephalic artery, left common carotid artery, and left subclavian artery and their branching ostia positions were quantified. Three-dimensional translation of vessel ostia, branching angles, and their changes secondary to cardiac pulsation and respiration were computed.During expiration, all ostia translated rightward from systole to diastole (P < .035). Regardless of cardiac phase, all ostia translated posteriorly and superiorly from inspiration to expiration (P < .05). Respiration induced greater posterior and superior translations than cardiac pulsation (P < .03). The left common carotid artery branch angled significantly more toward the aortic arch compared with the brachiocephalic artery and left subclavian artery (P < .03). No significant changes in branching angle were found from systole to diastole or inspiration to expiration.In patients with thoracic aortic aneurysms or dissections, the thoracic aortic arch translated significantly secondary to inspiration and expiration and to a lesser extent secondary to cardiac pulsation. Insignificant branching angle changes suggest that the aortic arch and its branch origins move predominantly in unison.

Abstract

Accurate quantification of changes in length, curvature, and bifurcation angles of coronary arteries due to cardiac motion is important for the design of coronary stents. A new method is developed to describe the dynamic characteristics of the human coronary artery. From cardiac-gated computed tomography (CT) data, 3-D surface geometry and centerline paths of the coronary arteries were constructed. For quantification of strain and twisting deformation, 3-D distortion-free vessel straightening and landmark matching algorithms were developed to compute the relative translation and rotation of distal landmarks with respect to a proximal landmark. For quantification of bending deformation, change in curvature was measured by computing a best-fit torus in the region of interest within a coronary segment. The optimal torus parameters were estimated by minimizing the standard deviation of distances from the surface mesh to the centerline of the torus. The angle between branch vessels was measured using linear fitting of centroid sets from the cross-sectional vessel lumen. The proposed methods were verified using a software phantom and applied to two patient specific CT datasets. Vascular deformations derived from these methods can provide information for designing bench-top tests for endovascular devices that better replicate the in vivo environment, thereby improving device performance prediction and leading to more durable designs.

Abstract

Abdominal aortic aneurysm (AAA) is a vascular disease resulting in a permanent, localized enlargement of the abdominal aorta. We previously hypothesized that the progression of AAA may be slowed by altering the hemodynamics in the abdominal aorta through exercise [Dalman, R. L., M. M. Tedesco, J. Myers, and C. A. Taylor. Ann. N.Y. Acad. Sci. 1085:92-109, 2006]. To quantify the effect of exercise intensity on hemodynamic conditions in 10 AAA subjects at rest and during mild and moderate intensities of lower-limb exercise (defined as 33 ± 10% and 63 ± 18% increase above resting heart rate, respectively), we used magnetic resonance imaging and computational fluid dynamics techniques. Subject-specific models were constructed from magnetic resonance angiography data and physiologic boundary conditions were derived from measurements made during dynamic exercise. We measured the abdominal aortic blood flow at rest and during exercise, and quantified mean wall shear stress (MWSS), oscillatory shear index (OSI), and particle residence time (PRT). We observed that an increase in the level of activity correlated with an increase of MWSS and a decrease of OSI at three locations in the abdominal aorta, and these changes were most significant below the renal arteries. As the level of activity increased, PRT in the aneurysm was significantly decreased: 50% of particles were cleared out of AAAs within 1.36 ± 0.43, 0.34 ± 0.10, and 0.22 ± 0.06 s at rest, mild exercise, and moderate exercise levels, respectively. Most of the reduction of PRT occurred from rest to the mild exercise level, suggesting that mild exercise may be sufficient to reduce flow stasis in AAAs.

Abstract

Hemodynamic conditions are hypothesized to affect the initiation, growth, and rupture of abdominal aortic aneurysms (AAAs), a vascular disease characterized by progressive wall degradation and enlargement of the abdominal aorta. This study aims to use magnetic resonance imaging (MRI) and computational fluid dynamics (CFD) to quantify flow stagnation and recirculation in eight AAAs by computing particle residence time (PRT). Specifically, we used gadolinium-enhanced MR angiography to obtain images of the vessel lumens, which were used to generate subject-specific models. We also used phase-contrast MRI to measure blood flow at supraceliac and infrarenal locations to prescribe physiologic boundary conditions. CFD was used to simulate pulsatile flow, and PRT, particle residence index, and particle half-life of PRT in the aneurysms were computed. We observed significant regional differences of PRT in the aneurysms with localized patterns that differed depending on aneurysm geometry and infrarenal flow. A bulbous aneurysm with the lowest mean infrarenal flow demonstrated the slowest particle clearance. In addition, improvements in particle clearance were observed with increase of mean infrarenal flow. We postulate that augmentation of mean infrarenal flow during exercise may reduce chronic flow stasis that may influence mural thrombus burden, degradation of the vessel wall, and aneurysm growth.

Abstract

Vessel deformations have been implicated in endoluminal device fractures, and therefore better understanding of these deformations could be valuable for device regulation, evaluation, and design. The purpose of this study is to describe geometric changes of the superficial femoral artery (SFA) resulting from hip and knee flexion in older subjects.The SFAs of seven healthy subjects aged 50-70 years were imaged with magnetic resonance angiography with the legs straight and with hip and knee flexion. From geometric models constructed from these images, axial, twisting, and bending deformations were quantified.There was greater shortening in the bottom third of the SFA than in the top two thirds (top, 5.9% +/- 3.0%; middle, 6.7% +/- 2.1%; bottom, 8.1% +/- 2.0% [mean +/- SD]; P < .05), significant twist in all sections (top, 1.3 degrees /cm +/- 0.8; middle, 1.8 degrees /cm +/- 1.1; bottom, 2.1 degrees /cm +/- 1.3), and greater curvature increase in the bottom third than in the top two thirds (top, 0.15 cm(-1) +/- 0.06; middle, 0.09 cm(-1) +/- 0.07; bottom, 0.41 cm(-1) +/- 0.22; P < .001).The SFA tends to deform more in the bottom third than in the other sections, likely because of less musculoskeletal constraint distal to the adductor canal and vicinity of knee flexion. The SFAs of these older subjects curve off axis with normal joint flexion, probably resulting from known loss of arterial elasticity with age. This slackening of the vessel enables a method for noninvasive quantification of in vivo SFA strain, which may be valuable for treatment planning and device design. In addition, the spatially resolved arterial deformations quantified in this study may be useful for commercial and regulatory device evaluation.

Abstract

To quantify in vivo deformations of the abdominal aorta and common iliac arteries (CIAs) caused by musculoskeletal motion.Seven healthy subjects (age 34+/-11 years, range 24-50) were imaged in the supine and fetal positions (hip flexion angle 134.0 degrees +/-9.7 degrees ) using contrast-enhanced magnetic resonance angiography. Longitudinal strain, twisting, and curvature change of the infrarenal aorta and CIAs were computed. The angle between the left and right CIAs and translation of the arteries were also computed.Maximal hip flexion induced shortening (5.2%+/-4.6%), twisting (0.45+/-0.27 degrees /mm), and curvature changes (0.015+/-0.007 mm(-1)) of the CIAs. The angle between the CIAs increased by 17.6 degrees +/-8.6 degrees . The iliac arteries moved predominantly in the superior direction relative to the aortic bifurcation, which would induce compression and bending, thus increasing curvature and angle between the CIAs. The abdominal aorta also exhibited shortening (2.9%+/-2.1%) and twisting (0.07+/-0.05 degrees /mm) deformation associated with the hip flexion.Although this study was limited to a few healthy young adults, musculoskeletal motion, specifically hip flexion, caused significant in vivo morphological changes (shortening, twisting, and bending) of the arteries. Predominant superior translation of the CIAs was observed, which suggests that preclinical testing of cyclic superior-inferior translational motion may aid in predicting stent-graft fractures. In turn, stent-graft design could be improved, decreasing overall stent-graft-related complications.

Abstract

The knowledge of dynamic changes in the vascular system has become increasingly important in ensuring the safety and efficacy of endovascular devices. We developed new methods for quantifying in vivo three-dimensional (3D) arterial deformation due to pulsatile and nonpulsatile forces. A two-dimensional threshold segmentation technique combined with a level set method enabled calculation of the consistent centroid of the cross-sectional vessel lumen, whereas an optimal Fourier smoothing technique was developed to eliminate spurious irregularities of the centerline connecting the centroids. Longitudinal strain and novel metrics for axial twist and curvature change were utilized to characterize 3D deformations of the abdominal aorta, common iliac artery, and superficial femoral artery (SFA) due to musculoskeletal motion and deformations of the coronary artery due to cardiac pulsatile motion. These illustrative applications show the significance of each deformation metric, revealing significant longitudinal strain and axial twist in the SFA and coronary artery, and pronounced changes in vessel curvature in the coronary artery and in the inferior region of the SFA. The proposed methods may aid in designing preclinical tests aimed at replicating dynamic in vivo conditions in the arterial tree for the purpose of developing more durable endovascular devices including stents and stent grafts.

Abstract

To examine the effects of swallowing and side-to-side head turning on stents in the internal carotid artery.Seven patients (4 men; mean age 76.9 years) who underwent carotid artery stenting for the treatment of atherosclerotic cervical carotid artery disease were examined with cine fluoroscopy. Geometric processing techniques were used to quantify carotid stent deformations due to head turning and swallowing forces. The variables measured included radial, axial, and crush deformations, as well as radii of stent curvatures during tested maneuvers.Radial deformations of the stented vessels were significantly less than axial and crush deformations, ranging from -10.2% to 15.5%. Axial deformations in response to both swallowing and head turning were positive (average 4.5%, range -14.5% to 14.1%), indicating a general lengthening of the stented vessel due to biomechanical motions. Crush strains exhibited the largest range of all of the deformation modes during both swallowing and head turning. Strain values ranged from -18.7% to 25.9% in the anteroposterior direction and from -25.6% to 21.9% in the lateral direction. Head turning produced fairly symmetrical crushing of the stent. Conversely, swallowing resulted in a preferential medial crush of the stented artery due to contraction of the pharyngeal constrictor muscles. Curvature measurements revealed a tightest radius of curvature of approximately 1.5 cm during ipsilateral head turning, with average values during both swallowing and head turning of approximately 10 cm.In general, head turning toward the stented artery produced greater deformation in the vessels than swallowing. Since patients are expected to undergo far more swallowing cycles than head turns, however, the accumulated deformations from swallowing may be more significant and should be considered in the design of fatigue resistant stents for carotid arteries.

Abstract

The authors describe an incident of a type I single strut fracture in a right renal artery (RRA) stent resulting in approximately 90% restenosis. Fracture was observed just distal to the ostium approximately 1 year after implantation in an 83-year-old man with a history of systemic cardiovascular disease. In addition, a statistical analysis of the clinically reported cases of left renal artery (LRA) and RRA stent fracture is provided, which suggests a greater susceptibility to fracture in LRA stents as demonstrated by the greater occurrence (67%) in the left side.

A Review of Peripheral Vascular Deformations Due to Respiration and Musculoskeletal InfluencesJournal of ASTM International (Symposium on Fatigue and Fracture of Medical Metallic Materials and Devices)Cheng, C.P.2008; 5 (10): JAI102074

Abstract

While relative lung perfusion distributions are cited in clinical decision making for congenital and acquired pulmonary vascular diseases, normal values and ranges have not been published for a large population of normally perfused lungs. These values of normal relative perfusion will be useful for establishing what is abnormal and for clinical decisions related to various pulmonary vascular diseases.Relative perfusion distributions were quantified for the top, middle, and bottom thirds of the right and left lungs with a semiautomatic algorithm in 206 normal scintigraphy lung studies (45 +/- 18 years, 149 female, 57 male) acquired between January 1, 2000 and March 30, 2004 in the Nuclear Medicine Division at Stanford Hospital and Clinics.The perfusion data were found to be highly non-Gaussian in nature (necessitating the use of Wilcoxon statistical comparisons), and the right/left perfusion ratio was found to be 52.5/47.5 (+/-2.1%) rather than the often quoted 55/45 split. While this right/left split was consistent between the genders, males had proportionally less perfusion in the lower left lung as compared with females (P < .05).The long-standing 55/45 right/left perfusion ratio assumption was found to be more than 1 standard deviation greater than the mean, and the population variance is very small. Relative pulmonary perfusion distribution varies significantly with lung region, gender, and age, and should be considered when making clinical decisions based on pulmonary perfusion.

Abstract

Localization of atherosclerotic lesions in the abdominal aorta has been previously correlated to areas of adverse hemodynamic conditions, such as flow recirculation, low mean wall shear stress, and high temporal oscillations in shear. Along with its many systemic benefits, exercise is also proposed to have local benefits in the vasculature via the alteration of these regional flow patterns. In this work, subject-specific models of the human abdominal aorta were constructed from magnetic resonance angiograms of five young, healthy subjects, and computer simulations were performed under resting and exercise (50% increase in resting heart rate) pulsatile flow conditions. Velocity fields and spatial variations in mean wall shear stress (WSS) and oscillatory shear index (OSI) are presented. When averaged over all subjects, WSS increased from 4.8 +/- 0.6 to 31.6 +/- 5.7 dyn/cm2 and OSI decreased from 0.22 +/- 0.03 to 0.03 +/- 0.02 in the infrarenal aorta between rest and exercise. WSS significantly increased, whereas OSI decreased between rest and exercise at the supraceliac, infrarenal, and suprabifurcation levels, and significant differences in WSS were found between anterior and posterior sections. These results support the hypothesis that exercise provides localized benefits to the cardiovascular system through acute mechanical stimuli that trigger longer-term biological processes leading to protection against the development or progression of atherosclerosis.

Abstract

The goal of this study was to quantify in vivo deformations of the superficial femoral artery (SFA) during maximum knee and hip flexion with use of magnetic resonance (MR) angiography to improve description of the complex, dynamic SFA environment.Contrast medium-enhanced MR angiography was performed on the leg vasculature of eight healthy adults in the supine and fetal positions. The SFA was defined as the centerline path of the iliofemoral segment from the profunda femoris to the descending genicular artery. Deformations that resulted from flexion from the supine position to the fetal position were quantified with the SFA path and its branches.Fourteen SFAs shortened from the supine position to fetal position, whereas two lengthened. Six of eight left SFAs twisted counterclockwise, and seven of eight right SFAs twisted clockwise. Straightness percentages for supine and fetal SFAs were 99.1%+/-0.4% and 98.7%+/-0.6%, respectively. From the supine position to the fetal position, the SFA shortened 13%+/-11% (P.8) between left and right limbs; however, no significant correlation existed for SFA twist angle.Complex and varying vascular and muscular anatomy among study participants made SFA lengths and deformations from the supine position to the fetal position unpredictable a priori; however, there were strong symmetries between left and right SFAs in terms of arc length, length change, and direction of twist. The data show that, from the supine position to the fetal position, the SFA tended to shorten and twist substantially, suggesting these as possible fracture mechanisms and also providing important parameters for stent design.

Abstract

To use MRI to quantify blood flow conditions in the proximal pulmonary arteries of healthy children and adults at rest and during exercise in an upright posture.Cine phase-contrast MRI was used to calculate mean flow and reverse flow index (RFI) in the main (MPA), right (RPA), and left (LPA) pulmonary arteries in healthy children and adults in an open-MRI magnet equipped with an upright MRI-compatible ergometer.From rest to exercise (150% resting heart rate), blood flow (liters/minute/m2) increased in the RPA (1.4+/-0.3 vs. 2.5+/-0.4; P<0.001), LPA (1.1+/-0.3 vs. 2.2+/-0.6; P<0.001), and MPA (2.7+/-0.5 vs. 4.9+/-0.5; P<0.001). RFI decreased in the LPA (0.040+/-0.030 vs. 0.017+/-0.018; P<0.02) and MPA (0.025+/-0.024 vs. 0.008+/-0.007; P<0.03). Adults experienced greater retrograde flow in the MPA than the children (0.042+/-0.029 vs. 0.014+/-0.012; P<0.02).It appears that at both rest and during exercise, in children and adults alike, RPA/LPA mean blood flow distribution is predominantly determined by distal vascular resistance, while retrograde flow is affected by proximal pulmonary bifurcation geometry.

Abstract

The prevalence of atherosclerosis in the abdominal aorta increases with age and is hypothesized to be related to adverse hemodynamic conditions including flow recirculation and low wall shear stress. Exercise has been shown to modulate these adverse conditions observed in the infrarenal aorta of healthy young subjects at rest. A custom magnetic resonance (MR)-compatible stationary cycle, an open MRI, and custom image processing software were used to quantify hemodynamic conditions in the abdominal aorta at rest and during cycling exercise in healthy subjects aged 50-70 years. The subjects increased their heart rate from 63+/-8 bpm at rest to 95+/-12 bpm during cycling exercise. Supraceliac blood flow increased from 2.3+/-0.4 to 6.0+/-1.4 l/min (P<0.001) and infrarenal flow increased from 0.9+/-0.3 to 4.9+/-1.7 l/min (P<0.001) from rest to exercise. Wall shear stress increased from 2.0+/-0.7 to 7.3+/-2.4 dynes/cm(2) at the supraceliac level (P<0.001) and 1.4+/-0.8 to 16.5+/-5.1 dynes/cm(2) at the infrarenal level (P<0.001) from rest to exercise. Flow and shear oscillations present at rest were eliminated during exercise. At rest, these older subjects experienced lower mean wall shear stress at the supraceliac level of the aorta and greater oscillations in wall shear stress as compared to a group of younger subjects (23.6+/-2.2 years). Compared to the younger subjects, the older subjects also experienced greater increases in mean wall shear stress and greater decreases in wall shear stress oscillations from rest to exercise.

Abstract

Compared with the abdominal aorta, the hemodynamic environment in the inferior vena cava (IVC) is not well described. With the use of cine phase-contrast magnetic resonance imaging (MRI) and a custom MRI-compatible cycle in an open magnet, we quantified mean blood flow rate, wall shear stress, and cross-sectional lumen area in 11 young normal subjects at the supraceliac and infrarenal levels of the aorta and IVC at rest and during dynamic cycling exercise. Similar to the aorta, the IVC experienced significant increases in blood flow and wall shear stress as a result of exercise, with greater increases in the infrarenal level compared with the supraceliac level. At the infrarenal level during resting conditions, the IVC experienced higher mean flow rate than the aorta (1.2 +/- 0.5 vs. 0.9 +/- 0.4 l/min, P < 0.01) and higher mean wall shear stress than the aorta (2.0 +/- 0.6 vs. 1.3 +/- 0.6 dyn/cm(2), P < 0.005). During exercise, wall shear stress remained higher in the IVC compared with the aorta, although not significantly. It was also observed that, whereas the aorta tapers inferiorly, the IVC tapers superiorly from the infrarenal to the supraceliac location. The hemodynamic and anatomic data of the IVC acquired in this study add to our understanding of the venous circulation and may be useful in a clinical setting.

Abstract

Many cases of muscular ischemia do not manifest without increased metabolic demand. Hence, diagnosis of intermittent claudication often requires inducing physiologic challenge, such as by exercise. Cine phase-contrast MRI can concurrently acquire cross-sectional vascular anatomy and through-plane blood velocities, enabling blood flow rate quantification. An MR-compatible stationary cycle was designed, constructed, and tested for flow quantification in large arteries during lower-limb exercise in a General Electric Signa SP 0.5 T open magnet. The cycle demonstrated smooth cycling during image acquisition, has freewheeling capability, is adjustable for subject size and strength, and can quantify workload. A healthy 59-year-old male was imaged at the supraceliac and infrarenal levels of the abdominal aorta at rest and during exercise. An exercise workload of 47.9 W was achieved. His heart rate increased from 52 to 78 bpm, supraceliac flow increased from 1.7 to 3.7 L/min, and infrarenal flow increased from 0.4 to 3.2 L/min from rest to exercise.

Abstract

Biologic variations between men and women have been hypothesized to contribute to the differences in atherosclerosis epidemiology of the two genders. Hemodynamics are also hypothesized to play an important role in the localization of atherosclerosis in the abdominal aorta. However, the hemodynamics of men and women have not been compared at this location at rest or during lower limb exercise conditions.A magnetic resonance-compatible exercise bicycle, magnetic resonance imaging techniques, and custom data processing software were used to quantify blood flow rate, wall shear stress, and oscillations in flow and wall shear stress at the supraceliac and infrarenal levels of the abdominal aorta of young healthy men and women at rest and during lower limb exercise.Heart rate increased from 73 +/- 6.2 bpm at rest to 110 +/- 8.8 bpm during exercise (P

Abstract

Arterial wall shear stress is hypothesized to be an important factor in the localization of atherosclerosis. Current methods to compute wall shear stress from magnetic resonance imaging (MRI) data do not account for flow profiles characteristic of pulsatile flow in noncircular vessel lumens. We describe a method to quantify wall shear stress in large blood vessels by differentiating velocity interpolation functions defined using cine phase-contrast MRI data on a band of elements in the neighborhood of the vessel wall. Validation was performed with software phantoms and an in vitro flow phantom. At an image resolution corresponding to in vivo imaging data of the human abdominal aorta, time-averaged, spatially averaged wall shear stress for steady and pulsatile flow were determined to be within 16% and 23% of the analytic solution, respectively. These errors were reduced to 5% and 8% with doubling in image resolution. For the pulsatile software phantom, the oscillation in shear stress was predicted to within 5%. The mean absolute error of circumferentially resolved shear stress for the nonaxisymmetric phantom decreased from 28% to 15% with a doubling in image resolution. The irregularly shaped phantom and in vitro investigation demonstrated convergence of the calculated values with increased image resolution. We quantified the shear stress at the supraceliac and infrarenal regions of a human abdominal aorta to be 3.4 and 2.3 dyn/cm2, respectively.